The daughter of a 33-year-old American woman who converted to Islam and was killed in fighting in Syria last week insisted her mother was "not a terrorist", although the dead woman's father said he had expressed concerns about her to the FBI.

Syrian state media said Nicole Mansfield, a single mother from Flint, Michigan, was killed with two others in an ambush by Syrian government forces on an opposition mission in northwestern Syria. Syrian media also showed the passport of a British man, Ali alManasfi, 22.

Security sources said it was not clear what Mansfield and alManasfi were doing or which rebel group they were with.

Insurgents are seeking to topple President Bashar al-Assad. The conflict has killed more than 80,000 people since March 2011.

Gregory Mansfield, Nicole Mansfield's father, told CNN on Friday that he approached the FBI three years ago after his daughter made remarks about Israel at an Easter family gathering.

"All I know is that I went to the FBI about my concerns," he said. "And I know they did follow up because they were following her for a while. They needed to revoke her passport, then this wouldn't be going on."

Lindsay Godwin, a spokeswoman at the FBI's Washington field office, said the bureau "is looking into the situation that happened in Syria".

In a posting on Facebook on Friday, Mansfield's daughter, Triana Lynn Mansfield, wrote: "My mother was NOT a terrorist.

"She went there for a reason that is unknown," she wrote. "But believe this - SHE WAS FORCED TO STAY."

The woman's grandmother Carole Mansfield said she was not convinced Nicole was in Syria to fight, although she had told her, "You're looking a rattlesnake in the face," after she converted to Islam and began wearing the hijab. "You never would have thought she'd be a terrorist," Carole Mansfield said.

"Nikki" Mansfield was a high-school dropout and teenage mother who was raised Baptist. Relatives said she appeared lost and wanted a cause to believe in.

In her late 20s, after an unsuccessful marriage, she converted to Islam after meeting Muslims online. She attended a local mosque but never showed signs of radicalism, her family said.

She married a Muslim immigrant after her conversion, but family members said they never met him and Nicole never talked about him.

"I don't even think they ever lived together," Carole Mansfield said. "He was a mystery. No one knew anything about him."

Britain's Guardian newspaper quoted a Syrian army officer as saying it was believed the third person killed was Canadian because his mobile phone listed numerous calls to Canada.

This article appeared in the South China Morning Post print edition as Slain US woman 'not a terrorist'